Chapter 3: Diagnosis of tuberculosis disease and drug-resistant tuberculosis

Figures & data

Figure 1. Algorithm for interpretation of T-SPOT®.TB assays.

T-SPOT product insert can be consulted for additional information and details: https://www.tspot.com/wp-content/uploads/2021/04/TB-PI-US-0001-V9.pdf.

Table 1. Number of bacteria seen on microscopy and laboratory interpretation.^a

Number of AFB seen by staining methods
Fuchsin stain (Kinyoun) (1000x magnification)^b	Fluorochrome (Auramine) (250x magnification)	Semi-quantitative grading system: Report
0 in 300 fields	0 in 30 fields	Negative
1-2 per 300 fields	1-2 per 30 fields	Inconclusive, repeat
1-9 per 100 fields	1-9 per 10 fields	1+ (rare)
1-9 per 10 fields	1-9 per field	2+ (few)
1-9 per field	10-90 per field	3+ (moderate)
>9 per field	>90 per field	4+ (numerous)

Abbreviation: AFB, acid-fast bacilli.

a From Canadian Tuberculosis Standards, 7th Edition. Public Health Agency of Canada.

b All reports should state staining method used and actual number of organisms observed.

Table 2. Summary of suggested turnaround times (refer to individual section for more information). ²⁸

Procedure	Turnaround time to completion/report
Specimen collection and arrival at the first laboratory	24 hours
AFB smear microscopy	24 hours from specimen receipt, where available
Nucleic acid amplification testing (NAAT) for M. tuberculosis detection	<7 days from smear result or 24 hours from receipt of specimen if AFB smear is not locally available
Identification of mycobacterial isolate as M. tuberculosis complex (or non-tuberculous mycobacteria)	2 days from positive culture
Primary phenotypic susceptibility testing	3 weeks from a positive culture received in reference laboratories, depending on antibiotic
Reporting of all test results	24 hours from test completion

Abbreviation: AFB, acid-fast bacilli.

Table 3. QuantiFERON®-TB Gold Plus interpretive criteria (manufacturer’s recommendations).

Nil (IU/mL)	TB1 and/or TB2 antigen minus nil (IU/mL)	Mitogen minus nil (IU/mL)	QuantiFERON®-TB (IU/mL)	Report/Interpretation
≤ 8.0	<0.35	≥ 0.5	Negative	M. tuberculosis infection not likely
	≥ 0.35 and < 25% of Nil value	≥ 0.5
	≥ 0.35 and ≥ 25% of Nil value	Any	Positive	M. tuberculosis infection likely
	<0.35	<0.5	Indeterminate	Results are indeterminate for TB-Antigen responsiveness. Please repeat if clinically indicated.
	≥ 0.35 and < 25% of Nil value	<0.5
> 8.0	Any	Any

Association of Public Health Labratories. Mycobacterium tuberculosis:Assessing Your Laboratory. 2019.

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